This invention relates to a planarization and overtravel control citcuit and the use of such a circuit in a multiprobe test system that utilizes a z-axis detector assembly having a plurality of data-detector probes for sensing surface contacts in the z-axis, detecting surface edges, enabling the monitoring of planarization, and controlling overtravel into the surface of a semiconductor slice.
In the formation of electronic circuits, integrated circuits may be fabricated from thin semiconductor slices having a multiplicity of matrices or microcircuits thereon. The general practice is for each slice to contain a multiple of identical repeating matrices of the same type of microcircuits. The individual unit or circuit is sometimes referred to as an integrated circuit chip or an individual bar.
Before distribution, the present practice is to test each of the circuits of each integrated chip on a slice or wafer prior to separating the slice into the desired integrated circuit components or combinations thereof.
Since each microcircuit or integrated circuit of each wafer is normally positioned in a predetermined precise relation with respect to adjacent circuit units, it is possible to test the circuitry if the probe can be accurately located on each preselected point that corresponds to the circuit to be tested. It is possible, for example, to test several different circuits at the same time on any one integrated circuit.
In the test procedure, there are several obstacles to overcome in order to have reliable testing without damaging the slice. One of the difficulties experienced in the use of testing probes which include a supporting body having a needle connected thereto, is that the point of the probe tip may form a scratch on the surface of the semiconductor wafer as it is contacted by the point. This is caused by the lack of an effective z-axis control. The z-axis is the direction established by the vertical movement of the chuck or moveable support of the semiconductor slice with respect to the probe tips. Among other things, the z-axis control is needed to compensate for surface warpage of slices which may be as much as 5 mils across the surface of a large slice, in determining the point of contact with the wafer, and in determining when the probe tips are off the slice, i.e. edge detection.
Semiconductor slice testing is performed on a multiprobe system such as the multiprobe system manufactured by Electroglas Corp., Menlo Park, California, model 1034X. The multiprobe machine contains a probe card which is a printed circuit board having attached thereto a series of data probes for injecting signals and collecting test data from the semiconductor slice. The present practice includes an edge sensor on the probe card which is similar to a data probe, but having an electrical switch mechanism. Operationally, the conventional edge sensor functions such that when the probe tip makes contact with the silicon slice, an electrical open is caused. This open is detected by the multiprobe system and allows for a continuance for testing procedures. When the chuck or support block for the semiconductor slice is vertically moved to make contact with the probe tips and no contact is detected by the edge sensor, the open condition will not occur and the multiprobe system will perform an indexing procedure and move the slice such that the data probes are over the next row of integrated circuit chips. This conventional edge sensor has been found to be unreliable in the art and the cause of a significant amount of down time of the multiprobe system and damage to chips which results when the chuck is continuously moved upward and probe tip contact is not identified. After the probe tips make contact with the slice, it is necessary for the chuck to move an additional 1-5 mils of overtravel in order to break through the oxide layer and make good electrical contact with the active circuit elements, a technique called scrub-in. If the probe contact with the slice is not identified, the overtravel cannot be controlled and the result is probe tip breakage, excessive overtravel, which damages chips and causes machine downtime.
One device for overcoming this basic problem of z-axis control may be found in co-pending application "Method and Apparatus for Determining Probe Contact", Lee Reid, Ser. No. 873,564 filed Jan. 30, 1978. In the latter co-pending application, a device is disclosed for edge-sensing and z-axis control. This device is in the form of a probe having a force sensitive material attached thereto such that when surface contact is made with the probe tip, the force distributed over the force sensitive material generates a signal that is detected by an external detector circuit and delivered to a multiprobe system. This device is used to sense the edge of a semiconductor slice having a plurality of integrated circuits thereon.
A further problem found in multiprobe test systems is the fact that the nature of the semiconductor slice is such that the integrated circuits defined thereon are in a patterned series of chips which may be rectangular in geometry. Due to the circular nature of the semiconductor slice, there results a series of partial integrated circuit chips on the edges of the semiconductor slice. The use of a single edge sensor will identify approximately half of the partial bars as a surface to be tested and the result is that the multiprobe system will attempt to test these partial bars, wasting time, identifying the partial bars to be a bad circuit, and thereby inking the partial bar for identification. The same problem is encountered in testing a broken semiconductor slice which gives rise to an even increased number of partial bars making edge detection of even greater importance.
Co-pending application "Four-Quadrant, Multiprobe Edge Sensor for Semiconductor Wafer Probing", Reid et al, Ser. No. 879,038 filed Feb. 21, 1978 presents a technique and apparatus for solving the partial bar testing problem. In the latter co-pending application, the single data-detector device is replaced with a four-quadrant data-detector device assembly. The four-quadrant edge sensing assembly includes a printed circuit board having a plurality of date probes physically and electrically connected thereto and further having four data-detector probes on the printed circuit board such that when testing a semiconductor slice, the four data-detector probes are found in each corner of an integrated circuit chip; partial integrated circuit chips may be detected as well as the edge of the slice. The signals generated by the force sensitive material found in each of the four data-detector probes are detected by a detector circuit having four channels which in turn deliver the signals to a multiprobe system which enables the identification of partial bars and indexing of the support block, to test the next row of integrated circuit chips.
Although the four-quadrant data-detector approach solves the partial bar and broken chip problem, there still exists a problem in the multiprobe test scheme with respect to monitoring the planarization of the data probes, identification of broken data probes or malfunctioning data probes, and control of the overtravel of the probe tip of the data probe into the surface of the semiconductor slice.
By placing a plurality of data-detector probes on an electrical support, as for example, a printed circuit board, it is possible to monitor the z-axis contact of each and every data-detector probe upon contact with the semi-conductor slice. The signals generated by the force sensitive material on each of the data-detector probes are delivered to a detector circuit which in turn will deliver relevant information to a multiprobe device and/or minicomputer for evaluation of the timing of the data-detector probe contact with the semiconductor slice surface to provide effective monitoring of the z-axis movement which will result in effective monitoring of planarization, control of overtravel, probe tip contact identification, and edge sensing. Such a system is disclosed in copending U.S. application Ser. No. 893,118 "Multiprobe Test System and Method of Using the Same", Lee R. Reid et al, filed Apr. 4, 1978.